This invention relates to intrusion detection systems and more particularly to an acoustic doppler motion detection system used to detect moving objects in a confined space.
Acoustic doppler motion detection systems are used in enclosed volumes such as rooms and the like and operate by transmitting or radiating a signal and monitoring the return signal for changes caused by motion of an object within the room. Many of the such rooms are reverberant and as such, under steady state conditions, produce a fixed standing wave pattern with nulls and peaks throughout the room. Because this standing wave pattern exists in the room, the level or magnitude of the signal at the receiving tansducer depends on the location of the latter relative to the pattern. If the location of contents of the protected space, such as crates in a warehouse of furniture in an office, remains unchanged indefinitely, the standing wave pattern is essentially stationary (unless there is a change in temperature) and the level of the received signal is substantially constant. However, if the contents are moved within the space or if the air temperature changes, the standing wave pattern changes so that the receiving transducer may be located at a null resulting in poor signal strength and degradation of system performance.
One technique that has been proposed to overcome this problem is the use of a distributed microphone such as an elongated electret cable or tape which is sensitive throughout its length. The rationale for this approach was that differences in signal strength due to the standing wave pattern would average out over the length of the microphone. In practice, however, this was not the case and the technique proved to be unsuccessful.
This invention is directed toward a solution of this problem.